1. Field of the Invention
The invention relates to an image forming apparatus and method. More particularly, the invention relates to an image forming apparatus such as copying machine, facsimile machine, printer, and the like for forming images utilizing an electrophotographic process.
2. Background
In recent years, in an image forming apparatus utilizing an electrophotographic process, a method for developing an electrostatic latent image on a photoconductive element by a dry type developing method using a drum-shape or belt-shape photoconductive element as an image bearing member is becoming widespread. The process produces a visible image by charging the photoconductive element by a charging device, forming a latent image by exposing the photoconductive element using an optical system including laser light, and developing the latent image by a developing device. When a sheet (hereinafter a sheet is a recording paper) jam or some other error occurs during the image forming process, typically the entire image forming apparatus is stopped.
However, because of the inertia of some components, developing can continue until the apparatus comes to a complete stop. This often results in needless consumption of toner. Especially in recent years, color copying machines have spread remarkably and some kinds of machines have a heavy flywheel or the like on a shaft of the photoconductive element to improve image quality by decreasing banding (a deviation in a line-to-line distance in a sub-scanning direction). In this kind of image forming apparatus, since a rotating shaft of the photoconductive element has a considerable inertial moment, it takes time until the photoconductive element stops when the entire image forming apparatus is commanded to stop during the image forming process. Therefore, even though each operation, such as a charging operation, a photoconductive element drive, a developing bias, and a developing device drive are commanded to stop at the same time, the area of the surface of the photoconductive element which is already charged (the area corresponding to a distance from a charging position to a developing position of the photoconductive element) passes through the developing station when the developing bias is off, and developer (a carrier and a toner) is delivered to the above-mentioned area of the photoconductive element.
In view of this, for example, according to Japanese Laid-Open patent Application No. 261963/1991, in response to an emergency condition instructions to stop development and to stop the photoconductive element are given at different times, considering the time difference between stopping times of the photoconductive element and the developing device (because their inertial moments differ, and there is a difference between stopping times of the motors). However, this may not be enough to prevent needless toner usage after turning on the apparatus again.
Further, in color copying machines which spread in recent years, an A.C. bias is superimposed on a D.C. bias for the purpose of improving image quality and developing ability. The developing bias method as described above tends to cause even more needless usage of toner in the case of an emergency stop of the image forming process.